1. Field of the Invention
The present invention relates to a virtual pressure sensor, and more particularly but not limited to, to a virtual pressure sensor for a so-called Common Rail (CR) electronic injection system equipped in an endothermic engine, and the following description is made with reference to this field of application for convenience of illustration only.
2. Description of the Related Art
As it is well known, strict restrictions imposed on the exhaust emissions and the fuel consumption of IC (Internal Combustion) engines by the European Union within 2005, in combination with recent developments of injection systems, are focusing the efforts of the automotive industry on the optimization of the injection process in IC engines.
It should be remembered that in a so-called CR (Common Rail) injection system, unlike conventional injection systems, the fuel is stored under pressure within an accumulator called a rail. Such a Common Rail injection system (hereinafter CR system) is shown schematically in FIG. 1, globally indicated as 1.
The CR system 1 thus comprises an accumulating tank, called a rail 2, which is connected in turn to an injection block 3 that includes a plurality of injectors 4. The rail 2 is further connected to a high-pressure pump 5 provided with a filter 6 and a precleaner 7.
The use of a pressure regulator realized by means of an electro-mechanical member for regulating the fuel pressure inside the rail 2 is known.
In particular, the injectors 4 are connected to the rail 2 through small pipelines 8, and include essentially a fuel nozzle or atomizer and a solenoid valve. The solenoid valve is energized by an ECU (Electronic Control Unit), generally shown with 9, arranged to activate the injection of fuel into the engine 10. The injection stops as the solenoid valve is de-energized.
The amount of fuel injected, under constant pressure, normally increases with the solenoid valve opening time, and is therefore wholly independent from the speed either of the engine 10 or of the pump 5.
In currently available systems, the duration of the fuel injection is established according to the pressure inside the rail 2 and to the desired amount of fuel (Qfuel) to be injected into a combustion chamber of the engine 10 to fill a possible demand for torque at a certain rotation speed of the driving shaft of the rotor itself. This correlative function is called mapping, and it is tabularly described as follows:
TABLE ISpeedRail pressureRotationDemandedregulator (dutyRail pressureInjectionDemand Qfuel(RPM)torque (Nm)%)(bar)duration (us)(mm3)100012031.862079024140014037.980072027180018050.8108074035.5220026068135094050.5260022068135094050.5
The rail pressure shown in Table I (Column 4) is not an actual pressure, but the pressure set by of the electronic unit 9 obtained by setting a duty cycle value as shown in the third column of Table I.
In particular, the pressure set depends linearly on the duty cycle value, as shown in FIG. 2.
The CR system 1 just described has a serious drawback in the very fact of considering a non-actual rail pressure, but rather the pressure set by the electronic unit 9, which is a slowly varying signal representing an average pattern, not the actual one.
The discordance between the pattern of the actual pressure value and that of the pressure set by the electronic unit 9 is revealed at once by suitably surveying the CR system 1 under normal conditions of operation of the engine 10, as shown schematically in FIG. 3, where the pattern of the pressure set in the rail (curve A) is significantly different from the actual pressure pattern (curve B).
This means that the duration of a single injection calculated according to the scheme of the CR system 1 will differ from that actually required for injecting the desired amount of fuel.
FIG. 3 also shows an injection profile, viz. the injector current law (curve C). It can be seen that the actual rail pressure signal inside the rail (curve B) undergoes fluctuations within a period of about 500 μs (microseconds), apparently related to single injections.
Such fluctuations, which may attain amplitudes of up to 120 bar, make current single-injection CR systems inaccurate, because the pressure is considered to be constant and equal to the value set by the regulator (curve A).
The above phenomenon is intensified in multiple injection systems, because the injections themselves are closer together within time.
The injection regulating accuracy provided by a preset constant pressure value is evidently low in latest generation multiple injection systems, and this is a limiting factor for optimum combustion, and accordingly, for top efficiency and minimized exhaust emissions.
It should be further noted that, through most of the main phase (MAIN) and throughout all the POST and AFTER phases, the actual pressure level is far different from the set level, viz. the level used for selecting the injection law. As a result, the amount of fuel injected into the chamber differs materially from the amount that has been planned in order to fill a certain demand for torque.
The underlying technical problem of this invention is to provide a pressure sensor for a so-called CR injection system with appropriate structural and functional features to optimize the amount of fuel injected into the engine at each injection, to reduce exhaust emissions, and to enhance the efficiency of the endothermic engine associated with said injection system, thereby overcoming the drawbacks with which current injection systems are beset.